WASHINGTON -- New research says the best way to protect society's most vulnerable from the flu: Vaccinate school-age children and their parents.

Kids already top the U.S. government's priority list for swine-flu shots this year because that new influenza strain targets the young. That's unusual, as flu usually is most dangerous to older adults.

But Thursday's study, in the journal Science, says vaccinating students should be a priority every year -- because schoolchildren are influenza's prime spreaders and their parents then are the virus' bridge to the rest of the community. The idea: Inoculating spreaders could create something of a cocoon around the people most at risk of flu-caused death.

Clemson University mathematical biologist Jan Medlock modeled what would happen if a virus like the ones that caused the 1918 and 1957 pandemics struck today. He tested multiple vaccination strategies against viruses of varying virulence to see which would give the best outcome for the least vaccine.

In typical winters, the U.S. has 85 million to 100 million doses of flu vaccine. If at least 40 million doses are available, then vaccinating children ages 5 to 19 and adults in their 30s -- their parents' average age -- gives society the most protection, Medlock and co-author Alison Galvani of Yale University reported.

In just one example, using a hypothetical flu strain as deadly as the notorious 1918 virus, the model predicted that deaths could be cut by more than half if just those ages are vaccinated, compared with vaccinating only the more usual targets: people over 50 and under 5.

Scientists also have begun to realize that flu vaccine doesn't work as well in people over 65 -- who account for most of the 36,000 flu-caused deaths each winter -- as it does in the young. While flu vaccine protects 75 percent to 90 percent of young healthy people, some research suggests the protection may plummet to 30 percent among their grandparents.